Parabolic reflector



y 1952 s. J. MASON 2,605,415

PARABOLIC REFLECTOR Filed Sept. 14, 1945 F|G.| '4 F|G.2

40 IPHERY O'6O 0.2 0.4 06 0.8 LOx L2 L4 1.6 L8 2.0

INVENTOR SAMUEL J4 MASON Qamz k kW ATTORNEY Patented July 29, 1 952 PARABOLIC REFLECTOR Samuel J. Mason, Boston, Mass, assignor, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Application September 14, 1945, Serial No. 616,410

This invention relates to directional microwave antennas having a paraboloid reflector, and particularly to paraboloid reflectors having a periphery so' shaped as to obtain maximum gain and, minimum side lobes for a given reflector aperture area and a given feed pattern.

For many purposes in connection with radio echo systems, it is desired that the antenna gain be very high in order to obtain a high degree of directivity of the radiated beam; and to avoid useless dissipationf of radiated energy and spurious directivity interpretations, it is desirable that theside lobes of the radiation beam pattern be minimized.

For the gain of a paraboloid antenna to be high the phase of radiation across the aperture should be constant. This implies that the wave front impinging on the paraboloid should be as nearly spherical as] possible. The center of this diverging spherical wave front called the electricalcenter ofthe feed is the point located at the-focus ofthe paraboloid. v

The size of side lobes is affected by the phase and intensity of illumination at the periphery of the reflector.- It is the object of this invention to provide an antenna having a reflector whose periphery is shaped to produce maximum gain and minimum side lobes for a given area of paraboloid. Another object is to construct a paraboloid refiectonhaving maximum efficiency for a given aperture area.

This invention results from the discovery that if the periphery of the paraboloid reflector is shapedjto follow a line of constant illumination intensity or; as it'may be called, an illuminationfconto'ur'li-ne, there results a'maximum of gain; and r ninimum of side lobe radiation for a given reflector area. With a primary feed possessing a known radiation pattern placed at a known point near the focus of the paraboloid,

and making a known angle with the axis of the paraboloid, the distribution of radiation intensity over the surface of the paraboloid can be calculated. The distribution of energy over a sphere centered on the feed, and of radius f is taken to be the focal length of the paraboloid, is first studied in any of several possible coordinate systems, and then the intensity distribution on the paraboloid surface may be calculated by attenuating the distribution on the sphere by the factor (f/r) 2 where "r is the radius vector from the focus to the paraboloid surface. Since there is no divergence of the radiation from the paraboloid surface to the paraboloid aperture, the intensity distribution in the aperture is also determined. In order that the paraboloid radia- 6 Claims. (01. 250 3.63)

* tour lines; 1

2 tion beam pattern may have low side lobes the periphery of the paraboloid should follow some one of the lower intensity contours of aperture illumination.

Other methods may be followed for ascertaining the location of the contour lines on any given paraboloid of revolution with a particular feed located at a known position and attitude relative to the reflector, for example, by direct measurement. The essence of this invention is that by using a portion of such paraboloid out along one of the lower intensity illumination contour lines the desirable consequences of high gain and low side'lobes will result.

Fig. l is a section diagram of a horn feed and a spherical surface and a paraboloid surface for illustrating the explanation of calculation of constant illumination contour lines on the paraboloid; r r

Fig. 2 is a graph of constant illumination con- Fig. 3 is a perspective diagram showing contour lines on a paraboloid; and N Fig. 4 is a perspective view of a paraboloid with a periphery shaped to conform 'to a contour line, together with a feed'for illuminating the reflector. Y

Referring now to Fig. 1, there is represented a feed horn 5 (having in itself a directional characteristic pattern, not shown) positioned at the focus 1 of a paraboloidreflector 9 of focal length I, and inclined at an angle to the axis of the paraboloid 9. As a result of the feed pattern there will be a variation in the intensity of illumination of the reflector at different points, for example, ll, [2, l3, l4, [5 (in one vertical plane) corresponding to points 2|, 22, 23, 24, 25 respectively on a sphere centered at the paraboloid focus 1 and having its radius equal to the focal length f. The magnitudes of intensity at points 2|, 22, 23, 24 and 25 will be the equivalent of the feed pattern. tensities at the corresponding points on the paraboloid surface, it is necessary because of the divergence of the energy to attenuate by a factor of (f/r) where r is a parameter representing the distance, from focus to each particular point on the paraboloid surface. Since there is no divergence of the radiation from the paraboloid surface to the paraboloid aperture, the intensity on the aperture for corresponding points 3| 32, 33, 34, 35 is also determined thereby. By joining up points of equal intensity, lines of constant intensity of illumination or illumination contour lines are obtained as shown in Fig. 2.

Fig. '2 is a typical set of illumination contour To calculate therefrom the inlines showing a half pattern, and in this instance, the periphery of the paraboloid reflector shown in Fig. 4 was shaped to coincide substantially with the -14 db illumination line 40 the only deviation being to round off the end, as shown by dotted line 42, fornractical reasons.

Fig. 3 shows as a perspective sketch a paraboloid reflector 50 bearing on its surface a family of equal illumination contour lines 5|, 52, 53, 54, and 55, which result from a certain wave guide feed 56. If the paraboloid be shaped to conform to any one of these lines as for example line 5-4, the resulting reflector will have maximum area efliciency in the sense of highest gain and lowest side lobes for the area of reflector.

Fig. 4 shows an embodiment of the principle of this invention and includes a paraboloid reflector 60 with periphery 6i shaped in accordance with the above to conform to a contour line of constant illumination intensity. Also shown is the wave guide feed 63 for the same, inclined at an angle with the pa-raboloid axis so that the result is a nonsymmetrical reflector shape with reference to the paraboloid axis.

While a specific embodiment and application of the invention has been shown, the principle of the invention is entirely general, and therefore the invention should not be deemed limited to the particular embodiment shown.

What is claimed is;

1. A directional microwave antenna system comprising a reflector disposed entirely in the surface of 'a theoretical paraboloid, a feed for radiating electromagnetic wave energy in a predetermined pattern at a particular frequency, said predetermined radiation pattern being nonsymmetrical with respect to. the axis'of said paraboloid, said reflector having a periphery coincident with a constant intensity of illumination contour line of said predetermined radiation pat tern of said feed.

2. An antenna system comprising, a reflector and a feed for radiating electromagnetic wave energy in the direction of said reflector in a predetermined radiation -pattern at a particular frequency, said predetermined radiation pattern being nonsymmetrical with respect to the axis of said reflector, the periphery of said reflector being defined as the intersection in the surface of said reflector of a constant intensity of illumination contour line of said predetermined radiation pattern-of said feed.

3. A directional microwave antenna system comprising a reflector in the form of a portion of a paraboloid' of revolution and a feed disposed along the axis of said paraboloid for radiating electromagnetic wave energy toward said reflector along said axis in a predetermined radiation pattern at a particular-frequency, said predetermined radiation pattern being nonsymmetrical with respect to said axis, said reflector having a periphery coincident with a contour line of constant illumination intensity of said predetermined radiation pattern of said feed.

4. An antenna comprising a reflector having an axis of directivity, a feed for radiating electromagnetic. wave energy a predetermined pattern at a particular frequency, said feed being positioned so as to illuminate said reflector, said radiation pattern being other than a solid of revolution about the axis of directivity of said reflector, said reflector having a periphery coincident with a preselected constant intensity of illumination contour line of said radiation pattern at said particular frequency.

5. An antenna com rising a reflector having an axis of directivity, a feed for radiating electromagnetic wave energy in a predetermined pattern at a particular frequency, said radiation pattern having a major axis. .of directivity, said feed being positioned so as to. ilhzminate said reflector with said major axis of directivity of said feed at an angle to the. axis of directivity of said reflector, said reflector having a periphery coincident with a preselected constant intensity of illumination contour line of said radiation pattern at said particular frequency.

6. An antenna system comprising a reflector having a paraboloidalreflecting surface with an axis of directivity and. a focus disposed along said axis, a feed positioned at said focus for radiating electromagnetic wave energy in a predetermined pattern at a particular frequency, said radiation pattern having a major axis of direc- REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,370,053 Lindenblad Feb. 20,1945 2,409,183 Beck Oct. 15, 1946 2,422,184 Cutler June 17, 1947 2,427,005 King Sept. 9, 1947 2,482,158 Cutler Sept. 20, 1949 2,483,575 Cutler Oct. 4, 1949 2,489,865 Cutler Nov. 29, 1949 2,501,070

Martinelli Mar; 21, 1950 

